Display device with transflective electrode

ABSTRACT

A transflective color display having apertures in reflective electrodes through which light from a backlight ( 9 ) passes in the transmissive mode. The switching behavior for both the reflective and the transmissive mode is made identical by introducing a retardation plate ( 11 ). The transmission efficiency is further increased by using only a monochrome green mode in transmission.

[0001] The invention relates to a display device comprising a displaypanel with a first light-transmitting substrate provided with reflectivematerial, a second light-transmitting substrate and an electro-opticalmaterial between said two substrates.

[0002] Such display devices are used, for example, in (portable) displayscreens in hand-held telephones, organizers but also, for example, inautomotive applications.

[0003] A (transflective) display device of the type mentioned in theopening paragraph is described in IBM TDB Vol. 15, No. 8, pp. 2435-6. Inthe reflective state, ambient light is reflected by the reflectivematerial, in this case a reflective electrode (a partly covered mirror)of, for example, chromium or aluminium. In the transmissive state, theseelectrodes pass light, and in the reflective state they reflect incidentlight. The actual picture elements (characters) are provided on theelectrodes by means of etching.

[0004] To ensure that sufficient light can be passed in the transmissivestate, the mirror must not be thick (in the case of aluminium, forexample, thinner than 15 mm). It is very difficult to provide suchmirrors with sufficient accuracy. Variations in thickness cause largevariations in light transmission and, as a result, lead to non-uniformbehavior in both the reflective state and the transmissive state. In thecase of relatively large panels, the small thickness additionallyinfluences the drive behavior because the square resistance becomes toohigh.

[0005] Another problem arises if birefringent material, for exampletwisted nematic (liquid-crystal) material is used in such a displaydevice, because said material causes the transmission-voltage curve tobe different in the transmissive mode and in the reflective mode.

[0006] The present invention aims, inter alia, at obviating one or moreof the above-mentioned drawbacks.

[0007] To achieve this, a display device in accordance with theinvention is characterized in that at the location of picture elementsthe reflective material is provided with at least one aperture.

[0008] By providing the layer of reflective material with (an)aperture(s) (occupying for example up to 30% of the surface area),sufficient light from a light source (backlight) is passed, while, onthe other hand, the layer of reflective material (for example ofaluminium) has such a thickness now (for example approximately 250 nm)that thickness variations of a few nanometers caused by processvariations do not influence the uniformity of the display panel. Alsothe square resistance decreases considerably.

[0009] The above-mentioned apertures can be provided in individualpicture electrodes in accordance with a pattern. In another embodiment,the aperture defines the individual picture elements.

[0010] A preferred embodiment of a display device in accordance with theinvention is characterized in that the electro-optical material isswitchable between two states having a different birefringence, thedisplay panel is provided with polarizers and with a retardation foilbetween the first substrate and a first (back)polarizer. Particularly inthe case of panels based on (super)twisted nematic effect (S)TN, thevoltage dependence for the transmissive mode differs substantially fromthat for the reflective mode. For use in the reflective mode, a displaypanel is generally embodied so that, after passing a front polarizer,light of a(n) (average) wavelength λ is subject to a change inpolarization in the liquid crystal material, such that, dependent uponthe voltage, elliptically to circularly polarized light impinges on thereflecting electrode (retardation ¼λ). Dependent upon the drive voltage,after reflection a smaller or larger degree of extinction occurs at thelocation of the front polarizer. By providing a retardation filterbetween the first substrate and the first (back)polarizer, saidapertures (in the transmissive mode) pass light at the location of thereflector, which light is elliptically polarized and, in particular,circularly polarized. As a result, the black-state is optimallycorrected. Consequently, the voltage-dependence of the transmissive modeis practically identical to that of the reflective mode, so that the useof a single voltage region is sufficient, thus saving costs.

[0011] Depending on the circumstances, it may be sufficient to use agreen light-emitting light source as the backlight. Generally, thetransmissive mode is used during less than 5% of the above-mentionedapplications, so that it is hardly disturbing that in the transmissivemode not the entire color palette is used. This means that veryefficient green backlights can be employed.

[0012] Preferably, the apertures are situated at the location of greenpicture elements. The wavelength of the light source is preferablyadapted to the transmission peak of the green part of a color filterpresent in the display cell.

[0013] These and other aspects of the invention will be apparent fromand elucidated with reference to the embodiments described hereinafter.

[0014] In the drawings:

[0015]FIG. 1 is a plan view of a part of a display device in accordancewith the invention,

[0016]FIG. 2 is a variant of FIG. 1,

[0017]FIG. 3 is a cross-sectional view taken on the line III-III in FIG.1,

[0018]FIG. 4 is a cross-sectional view taken on the line IV-IV in FIG.2,

[0019]FIG. 5 shows the variation of the transmission and reflection as afunction of the voltage for various types of devices,

[0020]FIG. 6 is a plan view of a part of another display device inaccordance with the invention, and

[0021]FIG. 7 is a cross-sectional view taken on the line VII-VII in FIG.6, and

[0022]FIG. 8 is a variant of FIG. 7.

[0023] The Figures are diagrammatic and not drawn to scale. In general,like reference numerals refer to like parts.

[0024]FIG. 1 and FIG. 2 are schematic plan views, and FIG. 3 and FIG. 4are cross-sectional views of a part of a display device comprising anelectro-optical display cell, in this example a liquid crystal cell 1with a twisted nematic liquid-crystal material 2 which is sandwichedbetween two transparent substrates 3, 4 of, for example, glass, providedwith electrodes 5, 6. The electrode 5 is made of a light-transmittingmaterial, while the electrode 6 is made of a reflective or diffuselyreflective material, such as aluminium or silver. The thickness of thereflective material is chosen to be such (150-400 nm) that no light ispassed. To allow light originating from an illumination source(backlight) (9) to pass nevertheless in the transmissive state, thereflective electrode material is provided with at least one aperture.For example, the electrodes 6 are surrounded by apertures 10 (FIGS. 1,3) or provided with apertures 10′ (FIGS. 2, 4) which occupy, forexample, maximally 25% of the electrode surface. During operation in thereflective mode, incident light is now fully reflected by the electrodes6 and absorbed at the location of apertures 10, 10′, which leads to anincrease of the contrast, while during operation of the illuminationsource 9, the apertures 10, 10′ allow sufficient light to pass. As shownin FIG. 4, the electrodes 6 may be provided, if necessary, ontransparent (ITO) electrodes 12.

[0025] Different electro-optical effects may be applied, in particularliquid crystal effects, such as (S)TN, guest-host, PDLC, ECB,ferro-electrics etc. In this example, the device comprises twopolarizers 7, 8 whose directions of polarization are mutuallyperpendicular in this example. The device further includes orientationlayers (not shown) which orient the nematic liquid crystal material atthe inner walls of the substrates, in this example, in such a way thatthe liquid crystal layer has a twist angle of approximately 60 degrees.In this case, the liquid crystal material has a positive opticalanisotropy and a positive dielectric anisotropy. Consequently, if theelectrodes 5, 6 are erergized by an electric voltage, the molecules andhence the directors orient themselves towards the field. In FIG. 5, thecurve indicated by dashed lines shows the reflection-voltagecharacteristic of such a device. Incident light 13 is transformed at avoltage V₂′ to elliptically (preferably circularly) polarized lightwhich is reflected at the location of the reflecting electrode 6 andreaches the polarizer 7 as practically linearly (at right angles to thedirection of polarization of the polarizer 7) polarized light and isabsorbed (complete extinction). At a decreasing voltage across theliquid crystal cell, the birefringence increases until, at a voltageV₁′, the retardation of the liquid crystal layer is such thatpractically maximum reflection occurs. When the display cell is used inthe transmissive mode, the transmission-voltage characteristiccorresponds approximately to the continuous line in FIG. 5, if nospecial measures are taken.

[0026] In accordance with a further aspect of the invention, in thisexample, a retardation foil 11 is situated between the polarizer 8 andthe liquid crystalline material 2, which retardation foil convertslinearly polarized light passed by the polarizer 8 into ellipticallypolarized light, preferably, of the same ellipticity as the light which,in the reflective mode, is incident on the reflective electrode 6 at avoltage V₂′. In the present example, in which the polarizers cross eachother at right angles, a ¼λ plate is used as the retardation foil 11, sothat the light originating from the source 9 reaches the liquid crystallayer as circularly polarized light and the switching behavior(transmission-voltage curve) becomes practically identical to the dashedline shown in FIG. 5. In particular, V₂ becomes practically equal toV₂′, so that the curves in the region near complete extinction coincide.

[0027] To reproduce color images, the device of FIG. 3, 4 is providedwith a color filter 14. As mentioned in the opening paragraph, thetransmissive mode is generally used during less than 5% of the lifetime. A white light source (backlight) 9, which emits all colors of thespectrum is generally less efficient (in lumens per watt) in the greenportion of the spectrum where the eye is most sensitive. The colorfilter 14 absorbs light in a large portion of the spectrum, so thatabsorption of light from a white light source (backlight) increasesfurther. For this reason, a green light source, for example anelectro-luminescent source or an LED, is often used in specificapplications (particularly telephones, organizers) where the lightsource 9 is used comparatively rarely. In this case, the wavelength ofthe source 9 is adapted, for example, to the transmission peak of thegreen (part of the) color filter. If necessary, the green pictureelements may also be embodied, so as to be larger (1.3-2 times) than thered or blue picture elements to further increase the transmission. Ifnecessary, the green picture elements are embodied so as to formseparate rows of picture elements between rows comprising both red andblue picture elements.

[0028] In the Table below, the light output in the transmissive mode iscompared for 6,5″reflective display panels with 640 (×3)×240 pictureelements, having respectively, a green and a white light source. greenwhite backlight efficiency (lm/W) 6 4 light output at 100 mW (cd/m²)23.6 15.7 Colour filter transmission (%) 95% 50% light output display(cd/m²) 1.62 0 56 Power at 2 cd/m²(W) 124 mW 357 mW

[0029] In both cases, the apertures 10 occupy approximately 20% of theoverall surface area. As shown in the Table, the use of a green lightsource leads, under equal conditions, to a higher light output in thetransmissive mode. Therefore, in applications in which the transmissivefunction is less important, it is more favorable to choose a green lightsource.

[0030]FIG. 6 is a plan view of a part of a display cell with a greenpicture element whose surface area is 1.4 times the surface area of ablue or red picture element (whose surface areas are identical). Since,as mentioned above, the wavelength of the light source 9 is adapted tothe transmission peak of the green portion of the color filter, theapertures in the reflective (metal) layer 15 are situated only at thelocation of the green picture element. The overall surface area of theapertures 10 is approximately 28.5% of the overall surface area of thegreen picture element, so that for each of the three types of pictureelements (red, green, blue) the reflective surface area is the same.

[0031] In this example, the reflector is embodied so as to be a separatemetal layer 15 on which the (now light-transmitting) ITO pictureelectrode 16 is provided. A passivation layer 17 is situated between themetal layer 15 and the picture electrode 16. The other referencenumerals have the same meaning as in the previous examples. FIG. 8 showsa variant in which the color filter 14 is adjacent to the metal layer15. The other elements, such as polarizers, the backlight, a retardationfoil, if any, etc., are not shown in FIG. 7, 8.

1. A display device comprising a display panel with a firstlight-transmitting substrate provided with reflective material, a secondlight-transmitting substrate and an electro-optical material betweensaid two substrates, characterized in that at the location of pictureelements the reflective material is provided with at least one aperture.2. A display device as claimed in claim 1 , characterized in that thereflective material forms part of a picture electrode.
 3. A displaydevice as claimed in claim 1 , characterized in that the electro-opticalmaterial is switchable between two states having a differentbirefringence, the display panel is provided with polarizers and with aretardation foil between the first substrate and a first polarizer.
 4. Adisplay device as claimed in claim 3 , characterized in that theretardation foil comprises a ¼λ plate, and light incident from the sideof the second substrate, in one of the two states, reaches thereflective electrode material practically as circularly polarized light.5. A display device as claimed in claim 1 , characterized in that thedisplay device is provided, on the side of the first substrate, with alight source emitting green light.
 6. A display device as claimed inclaim 5 , characterized in that the apertures are situated at thelocation of green picture elements.
 7. A display device as claimed inclaim 5 , characterized in that green picture elements occupy a largersurface area than red or blue picture elements.
 8. A display device asclaimed in claim 5 , characterized in that a row of green pictureelements is situated between rows comprising both red and blue pictureelements.